This invention relates generally to television signal transmission and receiving systems and particularly to scrambled signal transmission formats for use in subscription or pay television applications.
In subscription television systems, signals are transmitted,either "over the air" or over a suitable cable network, in a scrambled form which renders the broadcast video information largely unviewable on a conventional television receiver. In order to view the video display, each subscriber is provided with a decoder for unscrambling the broadcast signals and for coupling the unscrambled signals to a television receiver. A major consideration in designing a scrambling system is security to prevent unauthorized use of the broadcast information by non-paying subscribers. On the other hand, the scrambled signals must be capable of reliable decoding by authorized system subscribers and the system must be reasonably economical.
In addition to security problems, a cable or subscription television operator must be concerned about material that is "privately" broadcast, to subscribers of a certain service or channel, being considered offensive or distasteful to non-subscribers of the service or channel. Since it is often only the scrambling that prevents non-subscribers from viewing the material, the scrambling system should be capable of providing a "depth of scrambling" that would effectively preclude a non-subscriber from being offended by "seeing through the scrambling" of the video display. This consideration is quite important when dealing with adolescent viewers, for example. In scrambling systems that operate at baseband frequencies, the display on the television receiver cathode ray tube may be readily "blanked," that is, have no video displayed thereon, for unauthorized channels. However, many decoding systems operate at only RF frequencies and in such systems, blanking of the video display is not readily accomplished. In some RF systems, the decoder includes a channel tuner that may be tuned by the decoder controller to another channel in the event that the channel to which the tuner is tuned by the subscriber is not an authorized channel. These authorization and control systems are well-known in the art. In many systems, the decoder is actually tuned to a so-called "homing channel" when it is turned "off." In reality, the decoder is "on" and remains active to accept control signals from the cable head-end. With RF systems, complete isolation of signals is almost impossible, and it is not uncommon for signals to still be coupled, albeit very weakly, to a television receiver, even when the decoder is "off." Although the display may be snowy, it may still be viewable. Even though the signal is scrambled, the depth of scrambling may be insufficient and offensive material may still be partially viewable to nonsubscribers. Accordingly, there is a need in the art for a television signal scrambling system that is economical, yet effective, even at RF frequencies.